This invention relates to sheet metal end closures for containers. More particularly, this invention relates to methods of forming an opening tab in the central panel of a sheet metal easy open container end closure.
Recently, sheet metal container end closures, such as those interfoldedly seamed, as by a double seam, onto rims of beer and beverage containers, have been provided with non-removable opening devices. Instead of providing a pull tab attached to a score line-defined removable panel portion in the end closure central panel, a non-removable opening tab is provided which closes and is depressable through a pre-cut opening in the central panel. The opening tab typically is formed by stretching a portion of the central panel, as by forming a bulge therein, cutting substantially the entire bulge from the central panel to leave an integral panel-to-bulge connecting hinge and to form an opening in the panel, vertically displacing the bulge marginal edge portion below the rim of the opening, and completely flattening the bulge into an opening tab, and/or working the rim, to move the tab marginal edge or the rim or both relative to one another, such that portions of the tab marginal edge underlap the rim. This method of forming the opening tab and underlap and the use of a suitable sealant material along the underlap is designed to provide a tab-to-rim hermetic seal and to prevent the tab from being blown off the end closure by container internal pressures which, for container products such as beer and carbonated beverages can be at least about 95 psi.
However, the aforementioned method does not provide a tab that performs as desired. One of the main reasons is that it has been difficult to obtain a uniform underlap between all peripheral portions of the tab marginal edge and the rim of the opening. When the bulge is flattened and/or the opening rim is worked such as by being raised from the central panel and then partially flattened, the centerpoint of the opening tab becomes horizontally displaced relative to the center point of the opening, and there is more underlap in the direction of displacement than in the direction from which the displacement occurred. For example, when the underlap between the rim of a circular opening and the marginal edge of an integral hinge-connected circular opening tab is obtained by flattening the rim, the hinge also tends to be flattened. Flattening the hinge horizontally displace the tab away from the hinge and sometimes causes excessive underlap diametrically opposite the hinge, and insufficient underlap adjacent the hinge. This causes tab blow offs, and leakers adjacent the hinge. Attempts to overcome this tab-to-rim displacement and non-uniform underlap problem have involved providing excessive underlap, i.e. more than is really needed, along the entire tab edge periphery to insure that even with the displacement, there will be sufficient underlap adjacent the hinge. However, providing this excessive underlap involves a waste of sheet metal material.
An object of this invention is to overcome the aforementioned and other problems by providing a method of forming an opening tab for closing an opening in the central panel of a sheet metal end closure wherein a bulge having a brim is formed in and cut from the central panel, the bulge is reformed into an opening tab by reducing the bulge height and the angle of its sidewall relative to the horizontal, and the opening tab brim is coined to further extend its periphery substantially horizontally outward such that it further underlaps the rim of the central panel opening.
Another object of this invention is to overcome the aforementioned and other problems by providing a method of forming an opening tab for closing an opening in the central panel of a sheet metal end closure wherein a bulge having an outwardly extending shoulder and a brim is formed in and cut from the central panel, the bulge is reformed into an opening tab by reducing the height of the bulge and ironing out the shoulder, to extend the brim periphery substantially horizontally outward such that it underlaps the rim.
Another object of this invention is to provide the aforementioned method wherein in addition to reforming the bulge into an opening tab, there is added the step of coining the brim to extend it further under the rim.
Another object of this invention is to provide the aforementioned methods which provide a uniform underlap of the tab marginal edge and the rim of the central panel opening.
Another object of this invention is to provide the aforementioned methods and uniform underlap which minimize tab blow offs and leakers and prevent wastage of sheet metal material.